U.S. Pat. No. 5,477,250 issued on Dec. 19, 1995 discloses a tandem type of direct printing apparatus. In the direct printing apparatus, four printing stations are disposed along a sheet moving direction. Each printing station comprises a toner carrier (or developing sleeve roller) retaining toner on its outer periphery, a backing electrode opposed to the toner carrier and a printing head disposed between the toner carrier and the backing electrode, the printing head having a plurality of apertures and a plurality of electrodes surrounding each aperture. On the outer periphery of the toner carrier in each printing station are retained toner having different colors, for example, magenta, cyan, yellow and black. The backing electrode of each printing station is electrically connected to a power source, thereby between the toner carrier and the backing electrode is formed an electric field for attracting the toner on the toner carrier and propelling it toward the backing electrode through the apertures of the printing head. Between the printing head and the backing electrode in each printing station is formed a passage for a sheet.
When an ON voltage is applied to the electrode of the printing head in the printing station positioned at the most upstream side in the sheet moving direction, for example, the magenta printing station, the toner attracting force due to the electric field between the toner carrier and the backing electrode propels the toner on the toner carrier through the apertures toward the backing electrode and adheres it to the sheet. When an OFF voltage is applied to the electrode of the printing head, the toner attracting force does not affect the toner on the toner carrier, whereby the toner is never propelled. Thus, when ON and OFF voltage applied to the electrode of the printing head are controlled on the basis of a desired image signal, a magenta image corresponding to the image signal is printed on the sheet. In the same manner, by controlling the ON and OFF voltage applied to the electrode of the printing head in each of the downstream printing stations a different color of image is laid on the previously printed image to form a desired image.
In the aforementioned tandem type of direct printing apparatus, as the images formed by the printing stations are overlaid on each other, it is necessary that each aperture of printing head of one printing station corresponds to that of the other printing stations and that the corresponding apertures between the printing stations are aligned on a line parallel to the sheet moving direction. However, each printing station is installed separately from each other. Therefore, the corresponding apertures between the printing stations are shifted in a direction perpendicular to the sheet moving direction (hereinafter referred as a main scanning direction) due to the installation error of the printing head of each printing station. As the position shift of the apertures in the main scanning direction results in color deviation of the image, it is not possible to obtain a clear image.
For example, as shown in FIG. 9, in the case that an installation error of 50 .mu.m exists between the first printing station 104a and the second printing station 104b which have six apertures 102 with a pitch of 42 .mu.m, a position shift or a color deviation of 50 .mu.m which is same as the installation error is caused between the first aperture 102 of the first printing station 104a and the first aperture 102 of the second printing station 104b. In order to eliminate such color deviation, after setting the printing head 106 of the second printing station 104b, the position of the printing head 106 can be adjusted with high precision so that the installation error become zero. However, as this adjusting work is very difficult, the accuracy obtained by the adjusting work is limited. Even though each printing station has been installed with high accuracy, a position shift will be caused due to environmental temperature or aging, resulting in color deviation.